Charged particle beam system

ABSTRACT

A charged particle beam system wherein the output of the secondary electron detector is detected while the retarding voltage is varied between the values for which the secondary electrons do not reach the sample and the values for which the secondary electrons reach the sample, and the surface potential of the sample is determined on the basis of the relationship between the retarding voltage and the detected output of the secondary electron detector.

BACKGROUND OF THE INVENTION

This invention relates to a charged particle beam system and moreparticularly to such a system wherein beam focusing is controlled bycontrolling the electric potentials distributed over the surface of thesample through the measurement of those electric potentials.

DESCRIPTION OF THE RELATED ART

A scanning electron microscope (SEM) and a focused ion beam (FIB)apparatus are examples of an apparatus which uses a charged particlebeam such as an electron beam or ion beam to inspect, measure and work asample. In these apparatuses, the beam of accelerated charged particlesis focused on the sample by controlling the retarding (decelerating)voltage applied to the objective lens or the sample; the chargedparticle beam is scanned in a pattern of raster; the secondary electronsemitted from the sample are detected in synchronism with the rasterscan; and the two-dimensional signals (image) representative of thesuperficial shape of the sample can be obtained. The charged particlebeam system of most recent appearance is provided with an auto-focusmechanism so as to obtain clear images.

If the sample is intensely charged and the amount of charge is unknown,the focal point shifts according to the amount of charge. Therefore, ineffecting the auto-focusing function, it is necessary to expand thevariable range of the exciting current for the magnetic objective lensor the retarding voltage. Consequently, the time required to completethe auto-focusing operation is lengthened, resulting in a problem thatthe measurement time is prolonged.

When the beam is to be focused on the surface of the sample, which ischarged, through the control of the magnetic objective lens, the focalpoint shift due to the charge on the sample surface must be adjusted byadjusting the exciting current for the magnetic objective lens. In thiscase, however, if the magnification is calculated on the basis of theadjusted exciting current, the calculated magnification does notcoincide with the true magnification.

Several methods have been proposed to solve the problem resulting fromthe electrification of the sample surface, that is, the problem that theauto-focusing operation is prolonged or that the calculatedmagnification is erroneous. Those methods are disclosed in, for example,patent documents WO2003/007330, JP-2001-52642, and WO99/46798. Accordingto the document WO2003/007330, the amount of electrification of thesample is measured by the electrostatic capacitance measuring devicewhen the sample is carried into the sample chamber. The documentJP-2001-52642 discloses the procedure wherein the amount of charge onthe sample surface is determined by using the electrostatic capacitancedetector installed in the sample chamber. Accordingly, the sample isirradiated by the charged particle beam such as an electron beam havinga desired kinitic energy while the retarding voltage is so controlled asto cancel those electric potentials over the sample surface which aremeasured by the electrostatic capacitance detector. The documentWO99/46798 proposes a scanning electron microscope which can eliminatethe adverse effects on the image due to the electrification of sample.

SUMMARY OF THE INVENTION

Indeed the above mentioned, electrostatic capacitance measuring devicecan rapidly measure the surface potential of sample, but it cannotmeasure the potential in a small area, i.e. the local potential, on thesample surface.

In order to measure the amount of the sample electrification with thesystem disclosed in the document JP-2001-52642, the sample must becarried near to the electrostatic capacitance detector. This result in adecrease in throughput and a need for an additional provision of acircuitry for controlling the electrostatic capacitance detector.

In the document WO99/46798, the electric potentials over the samplesurface are controlled in such a manner that while the secondaryelectrons are being driven off the sample surface by the primaryelectrons irradiating the sample surface, the output of the secondaryelectron detector becomes maximum. In order to employ this method in thecase where the sample surface is highly charged, the acceleratingvoltage for the primary electrons constituting the electron beam must beincreased to cause the beam to reach the sample surface. This method,therefore, does not meet the need of today that measurement shouldpreferably be made using as low an accelerating voltage as possible.Further, according to the document WO99/46798, since the condition inwhich the output of the secondary electron detector becomes maximum issearched for while the secondary electrons are being emitted, the damageto the sample surface due to the electrons hitting thereon isconsiderable and it is also difficult to determine the condition to givethe maximum output.

Furthermore, it has come to be known that there exists the phenomenon ofelectrification which is different from the conventionally known “waferelectrification”. Namely, samples have come to be known which, in themost recent semiconductor manufacturing process, although there is nosurface electrification of the wafer when the retarding voltage isabsent, the application of the retarding voltage causes the wafersurface to be charged electrically. The cause of this type ofelectrification is still unknown, but the phenomenon is supposed to beattributed to the change in the surface potential due to the influenceof electric or magnetic field existing at the very point of observation.In order to solve this problem, it is necessary to measure the surfacepotentials under the condition of electric or magnetic filedapproximating the condition for actual observation.

The object of this invention is to provide a system for measuring thepotentials over the sample surface rapidly with high precision while theretarding voltage is being applied to the sample.

The concrete methodology according to this invention is as follows.

When the retarding voltage is increased to decelerate the chargedparticles forming the charged particle beam and reaches the level atwhich the kinetic energy of the charged particles becomes nearly equalto 0 eV, the charged particles impinging on the sample surface can notgive rise to secondary electrons. Use is made of this phenomenon, thatis, the potentials over the sample surface are estimated by measuringthe retarding voltage which reduces to 0 eV the kinetic energy of thecharged particles reaching the sample surface.

Concretely described, the specific value of the retarding voltage whichminimizes the output of the secondary electron detector is obtainedbetween the value of the retarding voltage at which the charged particlebeam does not reach the sample surface and the value of the retardingvoltage at which the charged particle beam can reach the sample surface.Then the surface potential of the sample is determined depending on thespecific value of the retarding voltage. Accordingly, the chargedparticle beam having a desired energy is cast onto the sample bycontrolling the retarding voltage so as to cause the determined surfacepotential to be offset.

According to the above described constitution, the potentials over thesample surface are measured, and the measured potentials are offset,whereby the charged particle beam having a desired energy can be castonto the sample. Consequently, the magnification error does not occur,and measurement can be effected at correct magnifications.

Further, since the surface potentials are offset, the time required forauto-focus operation can be shortened. As a result, throughput can beimproved for a charged particle beam system which is used to inspectpatterns or to measure pattern dimensions.

Furthermore, since the charged particle beam is used, the amount ofcharge on and within a tiny area of the sample surface can be measured.

Other objects of this invention and other concrete structures accordingto the invention will be described in the following description of theembodiment of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the structure of a charged particle beamsystem as a first and a second embodiments of this invention;

FIG. 2 shows the relationship between the retarding voltage and theoutput of the secondary electron detector in case where there is nopotential over the sample surface;

FIG. 3 shows the relationship between the retarding voltage and theoutput of the secondary electron detector in case where there is apotential of −0.2 kV over the sample surface;

FIG. 4 is a flow chart for the measurement of the surface potential;

FIG. 5 graphically shows a procedure for calculating the surfacepotential;

FIG. 6 graphically shows the variation in the efficiency of thesecondary electron emission in terms of V_(r)−I characteristic;

FIG. 7 shows the large-area electrification of wafer;

FIG. 8 shows measurement points over a wafer at which the surfacepotential is measured:

FIG. 9 shows in flow chart the procedure of measuring the potential overthe wafer surface and the procedure of correcting the surface potential;

FIG. 10 shows in flow chart the procedure of measuring the potentialover the wafer surface; and

FIG. 11 shows the screen displaying the conditions for performing thesurface potential measurement.

DETAILED EXPLANATION OF THE INVENTION

A scanning electron microscope as a first embodiment of this inventionwill now be described with reference to the attached drawings.

FIG. 1 schematically shows a first and a second embodiments of thisinvention. In a scanning electron microscope (SEM), electrons emittedfrom an electron source 1 are accelerated by a primary electronaccelerating electrode 2 to which a voltage is applied from a primaryelectron accelerating voltage source 13; decelerated by a retardingvoltage applied to a sample 8, and converged by a magnetic objectivelens 7 to focus upon the surface of the sample. When the beam ofelectrons hits the sample, secondary electrons are emitted from thesample. The secondary electrons are then accelerated toward the electronsource (referred to also as electron gun) 1 by the retarding voltage.The accelerated secondary electrons hit a reflector 5 and the reflectedsecondary electrons are collected by a secondary electron detector 10.Accordingly, the output of the secondary electron detector 10 changesdepending on the amount of electrons caught by the detector 10.

Here, the scanning of electron beam by using the retarding voltage willbe described briefly.

In the operation of a scanning electron microscope or a focused ion beamapparatus, the irradiation energy of the electron beam is controlled byapplying a voltage to a semiconductor wafer so as to focus the beam ofthe charged particles on the wafer without damaging the elements in thewafer surface due to the beam irradiation. This voltage applied to thesemiconductor wafer is called “retarding voltage”.

The scanning electron microscope produces the shape of the surface of asample by scanning the electron beam on the sample surface by means of ascanning coil 6, transducing the output of the secondary electrondetector 10 into a brightness signal in synchronism with the scanning,and displaying the brightness signal on the screen of a picture tube asthe two-dimensional image of the sample surface.

According to this invention, the retarding voltage is controlled; thethus emitted secondary electrons are detected; the sample surfacepotential is calculated by a computer 11 according to the detectedsecondary electrons; the surface potential is estimated by a surfacepotential estimator 12; and the energy of the electron beam reaching thesample is adjusted to a desired value through the control of theretarding voltage.

To obtain a clear image of the sample surface, the beam of chargedparticles must be exactly focused on the surface. However, it isdifficult to exactly focus the beam on the sample surface due to theroughness of the surface or the electric charge existing in the surface.Therefore, the charged particle beam system of today incorporates anauto-focus mechanism therein. According to many auto-focus mechanismsever proposed, the focal point of the charged particle beam is changedby discretely changing the exciting current for the objective lens orthe retarding voltage, and the images of the sample surface are takenfor the several focal points. The images associated with the differentfocal points are processed with a focus evaluation filter (differential,second-order differential, Sobel, Laplacian, etc.) to produce theassociated focus evaluation images leading to the calculation of thefocus evaluation value (referred to also as sharpness). Here, theweighted sum, the average or the variance, of the focus evaluationimages can be used as a measure of the focus evaluation value. Thesesteps of operation are usually performed as an auto-focus operation. Thevalue of that exciting current or retarding voltage which gives themaximum of the focus evaluation value is regarded as the excitingcurrent or the retarding voltage that occurs when the beam is properlyfocused on the sample surface. Throughout this specification, theoperation of changing the exciting current for the objective lens or theretarding voltage is called “sweeping” and the range within which theyare swept is termed “sweeping width”.

It has been recently observed that some wafers remain kept at a certainfixed potential even after it is electrically grounded. This fixedpotential is attributed to the polarization of polar material in thephotoresist due to the friction generated during the process of resistcoating by using the spin coater, or ascribed to the electrification dueto plasma etching.

Especially in the case where the SOI (silicon on insulation) techniqueis employed, that is, where an insulation film is formed on a wafer andthen semiconductor patterns are formed on the insulation film, it iswell known that some wafers are electrified at a potential of severalhundreds of volts. Under such high electrification, there is cause aproblem that the auto-focus operation needs a relatively long time,resulting in the decrease in overall inspection throughput.

Further, when focusing is performed by the objective lens while thesample surface is electrically charged, the following problem isincurred. Whenever a charged particle beam system is used, themagnification is calculated on the assumption that the sample is notelectrified at all. If the proper focus is not attained, themagnification is calculated on the assumption that the height of thesample has been changed. In case of a scanning electron microscope,however, if the sample surface is negatively charged, the electron beamis decelerated by the negative charge so that the focal point isshifted. If the shift of the focal point is adjusted by adjusting theexciting current for the objective lens and if the magnification iscalculated depending on the adjusted exciting current, then thecalculated magnification is different from the true magnification. Inthe case where the charged particle beam system is used to measure thedimensions of the patterns, a problem is incurred that the rightdimensions cannot be determined.

The following embodiment of this invention will be proposed inconsideration of the above mentioned problem.

The principle for measuring the electric potential of the sample surfacewill be described in reference to FIG. 2. Here, it is assumed that thereis no electrification of the sample surface. Let the acceleratingvoltage for the electron beam and the retarding voltage be denoted by V₁and V_(r), respectively. As an electron is negatively charged, theretarding voltage is set negative. If V_(r) is given such a value thatV_(r)<−V₁, the primary electrons forming the beam cannot reach thesample and are turned back toward the electron gun before reaching thesample. Upon reaching the reflector 5, the primary electrons causesecondary electrons to be emitted from the surface of the reflector 5.The secondary electrons are then detected by the secondary electrondetector 10. The primary electrons which are turned back and hit thelower surface of the reflector 5 have a velocity equal to the velocityat which the primary electrons emitted from the electron gun just passthrough the aperture of the reflector 5 toward the sample. Accordingly,the amount of the secondary electrons emitted from the reflector 5 isconstant irrespective of the magnitude of the retarding voltage V_(r).Therefore, the output of the secondary electron detector 10 remainsalmost constant irrespective of the magnitude of the retarding voltageV_(r).

Under the condition that V_(r)=−V₁, where the accelerating voltageequals the retarding voltage, the kinetic energy of the primaryelectrons barely reaching the sample is 0 eV (zero electron volt). Inthis case, since the primary electrons cannot generate secondaryelectrons at the sample surface, the output of the secondary electrondetector 10 goes down.

In the case where V_(r)>−V₁, the primary electrons of the beam emittedfrom the electron gun hit the sample surface so that secondary electronsare emitted from the sample surface and detected by the secondaryelectron detector 10. Since the kinetic energy of the primary electronsreaching the sample surface increases as the retarding voltage V_(r) isincreased, the amount of the emitted secondary electrons increases asthe retarding voltage V_(r) is shifted in the positive direction.

As understood from the above description, If V_(r)<−V₁, the output ofthe secondary electron detector 10 remains constant irrespective of themagnitude of the retarding voltage V_(r); if V_(r)=−V₁, where there isno secondary electron emitted, the output of the secondary electrondetector 10 goes down; and if V_(r)>−V₁, where the energy of the primaryelectrons hitting the sample surface varies depending on the magnitudeof the retarding voltage, the amount of the secondary electrons emittedfrom the sample surface varies depending on the energy of the primaryelectrons so that the output of the secondary electron detector 10varies depending on the magnitude of the retarding voltage.

In the foregoing description, the case is explained where the samplesurface is not previously electrified. Now, description is made of thecase with reference to FIG. 3 where the sample surface is electricallycharged. In FIG. 3, the sample surface is kept at an electrostaticpotential of −0.2 kV. In this case, as compared with the case shown inFIG. 2, the sample surface is charged at −0.2 kV so that the potentialof the sample surface becomes −V₁ when V_(r)=−V₁+0.2 kV. As seen fromFIGS. 2 and 3, the V_(r)−I characteristic curve for the sample surfacewithout electrification must be shifted by 0.2 kV in the positivedirection along the abscissa V_(r) to obtain the V_(r)−I characteristiccurve for the charged sample surface. Namely, since the V_(r)−Icharacteristic curve shifts depending on the potential of the samplesurface, the potential can be determined by measuring the amount of theshift.

The method of measuring the potential of the sample surface will bedescribed in detail with reference to the flow chart shown in FIG. 4.First, the retarding voltage V_(r) is set at the initial voltage thatprevents the primary electrons from reaching the sample surface (S01).Here, it is necessary to set the initial voltage at an appropriate valuein consideration of the anticipated surface potential of the sample. Forexample, if it is known a priori that the sample surface is charged atabout 0.1 kV, it is required to determine the initial value of theretarding voltage such that V_(r)<−V₁−0.1 kV. Then, an image can beobtained by two-dimensionally scanning the beam of the primary electronsover the sample surface at an appropriate magnification (S02). Since thebrightness of the image is proportional to the output of the secondaryelectron detector, the average brightness of the image is calculated(S03). Now, the retarding voltage is increased by an amount of Step kV(S04) and the above described operation is repeated until the presetrepetitive number N is reached (S05, S06).

By sweeping the retarding voltage, the surface potential V_(sp) iscalculated on the basis of the average brightness of the images obtainedcorresponding to the swept retarding voltages (S07). Then, the retardingvoltage is controlled in such a manner that the beam of the primaryelectrons can hit the sample surface with a desired energy of V_(acc) eV(S08). Here, in consideration of the accelerating voltage V₁ for theprimary electrons and the surface potential V_(sp), the retardingvoltage V_(r) must be given a value such thatV _(r) =−V ₁ +V _(acc) −V _(sp)  (1)

In the description given for FIG. 4, V_(r) is increased positively, butit is also possible to decrease V_(r) in the negative direction.Moreover, a similar effect can be obtained by one-dimensionally scanningthe beam of the primary electrons over the sample surface, obtaining theresultant one-dimensional signals from the secondary electron detector,and using the average of the one-dimensional signals, instead ofobtaining the two-dimensional images in (S02) step.

Furthermore, the program to execute the operation according to this flowchart can be incorporated in a scanning electron microscope or a chargedparticle beam system.

According to the conventional auto-focus mechanisms described abovewhich serve to evaluate the sharpness of pattern, the edge enhancingtreatment typically known as the differential treatment must be employedin the evaluation of the obtained images. In order for the edgeenhancing treatment to be effective, the S/N ratio of the image must begreater than a certain minimum value so that the noise contained in theimage may not be enhanced. To keep the S/N ratio above a certain levelin a scanning electron microscope, the ratio is generally improved byusing the frame accumulation technique or by reducing the noise throughthe smoothing filter before the differential treatment. These measures,however, result in a problem that the operation of auto-focusing islengthened. On the other hand, according to this invention, the samplesurface potential can be measured even where there is no pattern on thesample surface. It is only necessary to obtain the average brightness ofthe acquired images. Accordingly, the auto-focus technique of thisinvention is little affected by noise and does not need to resort to theframe accumulation technique or the noise reduction through thesmoothing filter for improving the S/N ratio. As a result, theauto-focus mechanism according to this invention can operate morequickly than the conventional auto-focus mechanism wherein the sharpnessof pattern must be evaluated.

With reference to FIG. 5 is now described a method for deriving thesurface potential V_(sp) from the plural values of retarding voltageV_(r) and the average brightness of the images corresponding to theplural values (proportional to the output I of the secondary electrondetector). The specific data point which gives the minimum brightness isobtained on the basis of the output I of the secondary electron detectoracquired by sweeping the retarding voltage discretely. This data pointand two additional data points located before and after it, are used todetermine the curve of a quadratic function that passes the three datapoints. This method can provide high precision for the particular valueof the retarding voltage for which the output of the secondary electrondetector is minimized even where the retarding voltage is discretelychanged.

Another embodiment is described below with respect to the data pointsused to obtain such a quadratic function as mentioned above. When theprimary electrons cannot reach the sample surface, the output of thesecondary electron detector or the average brightness of images is aboutconstant. This constant value is, for example, set as a threshold. Thedata points whose values are less than this threshold are used todetermine the minimum value. Curve fitting using quadratic functions isperformed on these data points whose values are less than the threshold.Hence, there is obtained with high precision that value of the retardingvoltage for which the output of the secondary electron detector isminimized even where the retarding voltage is discretely changed.

There, however, is a case where only the acquisition of the value of theretarding voltage for which the output I of the secondary electrondetector is minimized is not always enough for the purpose, depending onthe difference in the material of sample. In the variable range ofretarding voltage where the primary electrons are turned back, theoutput of the secondary electron detector remains constant irrespectiveof the value of the retarding voltage or the difference in the materialof sample. On the contrary, in the variable range of the retardingvoltage where the primary electrons can reach the sample surface, sincethe amount of secondary electrons emitted from the sample depends on theefficiency of secondary emission of electrons, the V_(r)−Icharacteristic curve changes as seen in FIG. 6 if the efficiency ofsecondary emission of electrons is varied. It is therefore concludedthat the value of the retarding voltage for which the average brightnessI of images is minimized depends on the material of sample. Hence, anoffset voltage V_(off) is previously determined to offset the“deviation” of the retarding voltage caused depending on the material ofsample, and the resultant retarding voltage V_(r) should be adjusted insuch a manner thatV _(r) =−V ₁ +V _(acc) −V _(sp) +V _(off)  (2)In general, it is impossible to obtain a properly focused image eventhough the surface potential is determined by using the above describedmethod and the retarding voltage is controlled according to the aboveexpression (2). That is because while the sample surface potential dueto the charges on the surface can be offset by controlling the retardingvoltage, the focus deviation due to the difference in the height of thesample is yet to be offset. In order to properly focus the electron beamon the sample surface, the focal point must be controlled by, forexample, changing the exciting current for the magnetic objective lens.If the sample stage for holding the sample is capable of fine movementin the vertical direction, the height of the sample may be finelycontrolled by moving the sample stage vertically.

In this embodiment, the retarding voltage V_(r) is swept, but theaccelerating voltage V₁ for the primary electrons or both theaccelerating voltage V₁ and the retarding voltage V_(r) may be swept.For the surface potential can be determined by calculating the sum of orthe difference between, the accelerating voltage V₁ for the primaryelectrons and the value of the retarding voltage V_(r) for which theoutput of the secondary electron detector is minimized.

In this case, the determined surface potential may be superposed on thevoltage to be applied to the sample stage or the electrode foraccelerating the primary electrons, or shared in superposition betweenthe sample stage and the electrode.

Embodiment 2

The retarding voltage may be increased in the positive or negativedirection. If the retarding voltage is increased in the positivedirection starting at the value thereof for which no secondary electronis emitted from the sample, then such an advantage can be obtained asfollows. The value of the retarding voltage V_(r) for which the outputof the secondary electron detector is minimized has only to be known todetermine the surface potential. Therefore, the measurement can befinished at the time when the output of the secondary electron detectorbecomes minimum or when the data enough to effect the above mentionedcurve fitting have been collected. The primary electrons have not yethit the sample surface up to this time and the obtained advantage isthat the damage to the sample due to the irradiation thereof by theprimary electrons can be prevented and that the electrification of thesample due to the irradiation thereof by the primary electrons can alsobe avoided. Further, if the variable range of the retarding voltage islimited to the above mentioned region, the measurement can be completedin a short time, resulting in an improvement on the throughput.

Embodiment 3

Semiconductor circuit patterns are usually integrated in the surface ofa sample called a wafer. A critical dimension measurement SEM is widelyused, for example, to measure the width of a conductive line of such apattern. The basic structure of such a dimension-measuring SEM is alsoas shown in FIG. 1. As shown in FIG. 1, a wafer is fixed on the samplestage 15, the stage is moved so as to enable the beam of the primaryelectrons to irradiate the pattern to be observed, and the desired partof the pattern can be observed.

In the ordinary measurement of semiconductor circuit patterns, there isa case where several hundreds of measurement points in a single waferare subjected to a required measurement at the stage of trialfabrication. It often happens that even after mass production isstarted, several tens of points in a single wafer are subjected to aprescribed measurement. In such a case, the throughput is deterioratedif the surface potential measurement is performed at all the measurementpoints according to the method described above in Embodiment 1. Thedeterioration of the throughput can be prevented by actually measuringthe surface potentials at several points in a wafer before themeasurement of pattern dimensions and accordingly estimating the surfacepotentials at arbitrary measurement points on the basis of the actuallymeasured surface potentials.

In most cases, the entire surface of the wafer is electrified, with thecentral part of the wafer having a higher potential and the peripheralpart thereof having a lower potential, as shown in FIG. 7. It istherefore preferable to represent the distribution of the surfacepotentials approximately with such a function as shown in FIG. 7. Forexample, as shown in FIG. 8, the surface potential is measured at tenpoints on the wafer. Then, the surface potential V_(sp)(x, y) at anarbitrary coordinate point (x, y) can be approximated on the basis ofthe surface potentials obtained at the ten measurement points accordingto the following expression (3).V _(sp)(x, y)=a×r ³ +b×r ² +c×r+dr=(x ² +y ²)^(1/2)  (3), where x and y are the abscissa and the ordinate of the rectangularcoordinate system with its origin located at the center of the wafer,and a, b, c and d are constant coefficients.

The coefficients a, b, c and d may be determined through the use of theleast square method so that the resultant cubic function can best fitthe curve that approximates the data obtained by the measurement. Inthis example, the curve fitting function is derived on the assumptionthat the potential distribution is symmetric with respect to the centerof the wafer, but any other type of expression for approximation may beemployed if it is appropriate in approximating the potentialdistribution over the wafer surface. Moreover, the surface potential atan arbitrary point on the wafer may be determined by the interpolationon the basis of the data collected at the plural measurement points.

The dimension-measuring SEM enables an unattended operation inmeasurement by operating in accordance with a series of measuring stepsrecorded in a file referred to generally as a recipe. The measuringoperation according to the recipe will be described below with referenceto FIG. 9. In case where a single wafer is measured by using a recipe,the wafer is first carried into the sample chamber, and then thepotential over the wafer surface is measured. Namely, the firstmeasuring point on the wafer surface defined in the recipe is reached(S20) and the surface potential there is measured (S21). Suchmeasurement is repeated for all the measuring points defined in therecipe (S22). When the measurement of surface potential is completed,such a surface potential approximating function V_(sp)(x, y) as theabove expression (3) is calculated (S30).

At the beginning of the pattern dimension measuring steps, the firstmeasuring point is reached by the movement of the sample stage (S40).Thereafter, the surface potentials at the measuring points aredetermined by using the surface potential approximating functionV_(sp)(x, y) obtained in the step S30, the retarding voltage iscontrolled by using the above expression (2), and the effect of thesurface potential is offset at each measuring point (S41). After thesesteps, the auto-focus operation is performed (S42), the pattern to bemeasured is located (S43), and the dimensions of the targeted patternare finally measured (S44).

In general, the amount of electrification of wafer surface may changelargely depending on semiconductor manufacturing processes. It may alsohappens that different wafers have different amounts of surfaceelectrification even in the same manufacturing process. According to themethods described in the foregoing, since the surface potentials atplural measuring points must be determined even when there is little orno electrification in the wafer surface, then the measurement efficiencycannot be said to be very high. The electrification of wafer is normallygreater at the center of the wafer than at the periphery thereof. Thesurface potential is first measured in the wafer center to determinewhether the electrification in the wafer center is considerable or not.If the electrification in the wafer center is considerable, themeasurement of the wafer surface potential is performed, and if theelectrification in the wafer center is negligible, such measurement isomitted. Accordingly, the deterioration of the throughput can beprevented.

The flow of the above described series of steps is illustrated in FIG.10. As seen in FIG. 10, the steps S20 through S30 shown in FIG. 9 aremodified. Namely, the surface potential in the wafer center is firstmeasured in the wafer surface potential measuring procedure 1. As seenin FIG. 10, the surface potentials are measured at the measuring pointslocated within a circle of a predetermined radius (S50, S51, S52). Inthe case shown in FIG. 10, the surface potentials are measured at twomeasuring points. Thereafter, the surface potential near the wafercenter is estimated on the basis of the surface potentials determined inthe step S51. Here, it suffices to strike the average of the surfacepotentials determined at the two measuring points to obtain theestimated potential near the wafer center. Then, if the surfacepotential near the wafer center is lower than a predetermined threshold,the wafer surface is deemed to be “void of electrification” (S53). Whenthe judgment “void of electrification” has been passed, the flowproceeds to the pattern dimension measuring procedure. On the otherhand, if the surface potential near the wafer center is higher than thepredetermined threshold, the wafer surface is deemed to be“electrified”. When the judgment “electrified” is passed, themeasurement of surface potentials is further performed at additionalmeasuring points distributed over the sample surface (S54, S55, S56).Thereafter, the surface potential approximating function V_(sp)(x, y) isobtained (S30). In this case shown in FIG. 10, the additional surfacepotentials are measured at the eight remaining measuring points.

The screen image illustrating how the wafer surface potential ismeasured according to the recipe will now be described with reference toFIG. 11. The first data to be defined is the flag 19 indicating whetherthe surface potential measurement is performed or not. Since thepresence or absence of the surface electrification depends on samples,it should first be determined whether the surface potential measurementis performed or not. The second data to be defined is the sweeping range20 for the surface potential measurement. Of course, different sampleshave different surface potentials, but if the surface potentials at theselected measuring points are previously known to fall within a certainrange of values, the time required for the surface potential measurementcan be shortened by appropriately narrowing the sweeping range. Thethird data to be defined is the position of the measuring point wherethe surface potential measurement is performed. In the example shown inFIG. 11, the coordinates 21 in a chip are specified and the chip 22whose surface potential is to be measured is selected by the mousepointer 23. Since the chip whose surface potential is to be measured canbe selected by manipulating a mouse, the selection of the chip issimplified.

In the above described screen image, three independent pieces of dataare inputted. In the application of the sample surface potentialmeasuring technique according to this invention, as already described inEmbodiment 1, there should not necessarily be patterns in the samplesurface and therefore the surface potential measurement may be performedat any measuring point. Consequently, when a recipe is prepared, it ispossible to considerably reduce the number of the pieces of datainputted by a user. For example, only the number of the measuring pointsmay be defined while the measuring points where the surface potentialsare actually measured are randomly selected by the system itself.Alternatively, it is only specified whether the surface potential offsetis performed or not, and the surface potential measurement may beperformed at the plural coordinate points recorded previously in thesystem.

According to the above described method, several to ten measuring pointsare enough for the surface potential measurement and therefore thedeterioration of throughput can be limited to the minimum. Further,since the wafer surface potential can be appropriately controlled, thesweeping range for the auto-focus operation can be narrowed so that thetime required for the auto-focus operation can be shortened. Even incase where the wafer surface potential is measured, if there are manymeasuring points for pattern dimension measurement, the auto-focusoperation at each measuring point can be omitted or the time requiredtherefore can be shortened. Consequently, the time required forexecuting the recipe can be shortened. Furthermore, since the surfacepotential is controlled, the error of magnification arising from thesurface electrification can be eliminated and the correct measurement ofdimensions can be secured.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A charged particle beam system comprising a source of chargedparticles; a charged particle detector for detecting charged particles;a sample stage for supporting a sample; and a power source for applyinga variable voltage to the sample stage, wherein the voltage to beapplied to the sample stage is varied between the values for which thesecondary electrons do not reach the sample and the values for which thesecondary electrons reach the sample; the output of the charged particledetector is measured while the voltage to be applied to the sample stageis being varied; and the surface potential of the sample is determinedon the basis of the relationship between the varied voltage and thedetected output of the secondary electron detector.
 2. A chargedparticle beam system as claimed in claim 1, wherein a one-dimensionalsignal or a two-dimensional image is obtained as the output of thecharged particle detector; the amplitude of the one-dimensional signalor the brightness of the two-dimensional image is obtained; therelationship between the voltage applied to the sample stage and theamplitude of the one-dimensional signal or the relationship between thevoltage applied to the sample stage and the brightness of thetwo-dimensional image, is obtained; and the surface potential of thesample is determined on the basis of the relationship.
 3. A chargedparticle beam system as claimed in claim 1, wherein that value of thevoltage applied to the sample stage for which the output of the chargedparticle detector is minimized, is obtained on the basis of therelationship between the voltage applied to the sample stage and theoutput of the charged particle detector; and the surface potential ofthe sample is determined depending on the charged particle beamaccelerating voltage.
 4. A charged particle beam system comprising asource of charged particles; a charged particle detector for detectingcharged particles; a sample stage for supporting a sample; and a powersource for applying a variable voltage to the sample stage, wherein thevoltage to be applied to the sample stage is varied; the output of thecharged particle detector is measured; and the surface potential of thesample is determined by using that value of the voltage applied to thesample stage for which the output of the charged particle detector islower than a predetermined threshold.
 5. A charged particle beam systemas claimed in claim 4, wherein a one-dimensional signal or atwo-dimensional image is obtained as the output of the charged particledetector; the amplitude of the one-dimensional signal or the brightnessof the two-dimensional image is obtained; and the surface potential ofthe sample is determined by using that value of the voltage applied tothe sample stage for which the amplitude of the one-dimensional signalor the brightness of the two-dimensional image is less than apredetermined value.
 6. A charged particle beam system as claimed inclaim 4, wherein that value of the voltage applied to the sample stagefor which the output of the charged particle detector is minimized, isobtained; and the surface potential of the sample is determineddepending on the accelerating voltage for the beam of the chargedparticle.
 7. A charged particle beam system as claimed in claim 1,wherein the obtained surface potential is superposed on the voltageapplied to the sample stage.
 8. A charged particle beam system asclaimed in claim 4, wherein the obtained surface potential is superposedon the voltage applied to the sample stage.
 9. A charged particle beamsystem as claimed in claim 1, wherein the surface potential at anarbitrary point on the sample surface can be estimated on the basis ofthe relationship between the preselected measuring points on the sampleand the surface potentials determined at the preselected measuringpoints.
 10. A charged particle beam system as claimed in claim 9,wherein the estimated surface potential is superposed on the voltageapplied to the sample stage so as to control the sample surfacepotential; and the charged particle beam is so controlled that it mayhit an arbitrary position on the sample surface with a desired kineticenergy.
 11. A charged particle beam system as claimed in claim 9,wherein the surface potentials are measured at measuring points within acircular area having a predetermined radius, located in the center ofthe sample, in case of measuring surface potentials at plural measuringpoints distributed over the sample surface; further surface potentialmeasurement is ceased when the measured surface potentials do not exceeda preset threshold; and the further surface potential measurement iscontinued when the measured surface potentials exceed the presetthreshold.
 12. A charged particle beam system as claimed in claim 1,wherein the variable range of the voltage applied to the sample stage isspecified.
 13. A charged particle beam system comprising a source ofcharged particles; an accelerating electrode for accelerating a chargedparticle beam emitted from the source of charged particles; a firstpower source for applying a variable voltage to the acceleratingelectrode; a charged particle detector for detecting charged particlesemitted from a sample irradiated with the charged particle beam; asample stage for supporting a sample; and a second power source forapplying a variable voltage to the sample stage, wherein the kineticenergy of the charged particle beam is varied by controlling the voltageof the first power source and/or the voltage of the second power source;the output of the charged particle detector is detected while thekinetic energy of the charged particle beam is being varied; and thepotential of the sample surface is determined by using the value of thevoltage of the second power source for which the output of the chargedparticle detector remains less than a preset threshold.
 14. A chargedparticle beam system as claimed in claim 13, wherein the determinedsurface voltage is superposed separately on the voltage applied to thesample stage or the voltage applied to the accelerating electrode, ordistributively on both the voltage applied to the sample stage and thevoltage applied to the accelerating electrode.
 15. A charged particlebeam system as claimed in claim 14, wherein the surface potential at anarbitrary point on the sample surface can be estimated on the basis ofthe relationship between the preselected measuring points on the sampleand the surface potentials determined at the preselected measuringpoints.
 16. A charged particle beam system as claimed in claim 15,wherein the determined surface voltage is superposed separately on thevoltage applied to the sample stage or the voltage applied to theaccelerating electrode, or distributively on both the voltage applied tothe sample stage and the voltage applied to the accelerating electrode,so as to control the surface potential of the sample; and the chargedparticle beam is so controlled that the beam may hit the sample at anarbitrary position on its surface with a desired kinetic energy.
 17. Acharged particle beam system as claimed in claim 15, wherein, for themeasurement of the surface potentials at plural measuring pointsdistributed over the sample surface, the surface potential measurementis initially performed at plural measuring points within a circular areahaving a predetermined radius with its center located at the center ofthe sample; and further surface potential measurement is ceased if thesurface potentials obtained within the circular area do not exceed apreset threshold, whereas the further surface potential measurement iscontinued at the remaining measuring points distributed over the samplesurface if the obtained surface potential exceeds the preset threshold.18. A method for measuring the surface potential of a sample, whereinthe kinetic energy of the charged particle beam is varied; the electronsemitted from the sample irradiated with the charged particle beam aredetected; the accelerating voltage for accelerating the charged particlebeam and the voltage applied to the sample stage are detected when theamount of the emitted electrons becomes minimum; and the surfacepotential of the sample is determined on the basis of these detectedvoltages.
 19. A charged particle beam system as claimed in claim 18,wherein the determined surface potential of the sample is superposed onthe retarding voltage or the accelerating voltage.
 20. A program forissuing to a computer used with a charged particle beam system, acommand for changing the kinetic energy of the emitted charged particlebeam; a command for detecting the secondary charged particles emittedfrom the sample irradiated with the charged particle beam; a command formeasuring the accelerating voltage for the charged particle beam and thevoltage applied to the sample stage when the amount of the detectedsecondary charged particles becomes minimum; and a command fordetermining the surface potential of the sample on the basis of the sumof these voltages.
 21. A program as claimed in claim 20, for furtherissuing a command for superposing the determined surface potential onthe voltage applied to the sample stage or the accelerating voltageapplied to the accelerating electrode.
 22. A charged particle beamsystem comprising a source of charged particles; a charged particledetector for detecting charged particles; a sample stage for supportinga sample; and a power source for applying a variable voltage to thesample stage, wherein the voltage applied to the sample stage isincreased in the positive direction, starting at the values for whichthe charged particle beam does not reach the sample; the output of thecharged particle detector is measured while the voltage applied to thesample stage is being varied; and the surface potential of the sample isdetermined on the basis of the relationship between the varied voltageof the sample stage and the output of the charged particle detector.